It is found that the PSSHG increases with increasing Cu content up to 0.6% and then decreases due to the formation of the Cu-agglomerates.. The PSSHG for the crystal with Cu content high
Trang 1N A N O P E R S P E C T I V E S
of Cu-Agglomerates
M Idrish Miah
Received: 12 September 2008 / Accepted: 11 November 2008 / Published online: 22 November 2008
Ó to the authors 2008
Abstract Cu-doping effects in CdI2 nanocrystals are
studied experimentally We use the photostimulated second
harmonic generation (PSSHG) as a tool to investigate the
effects It is found that the PSSHG increases with
increasing Cu content up to 0.6% and then decreases due to
the formation of the Cu-agglomerates The PSSHG for the
crystal with Cu content higher than 1% reduces to that for
the undoped CdI2crystal The results suggest that a crucial
role of the Cu-metallic agglomerates is involved in the
processes as responsible for the observed effects
Keywords Nanocrystals Defects Surface properties
Electron–phonon interaction
Introduction
Nonlinear spectroscopy and photostimulated second
har-monic generation (PSSHG) are the two important tools to
investigate the higher-order nonlinear optical processes, in
particular, in semiconductors [1] The PSSHG is prevented
by symmetry in a centrosymmetric material process So, in
order to observe the PSSHG, one needs to have a
noncentrosymmetric process Fortunately, there are differ-ent ways to enhance the PSSHG These include (1) the reduction of the size of the crystals to the nanometer scale, (2) lowering the crystal temperature and (3) insertion of suitable impurities into the crystal with an appropriate amount [1] The nanometer-sized crystals take into account the quantum-confined effect (quantum confinement domi-nates the material’s electronic and optical properties), where k-space bulk-like dispersion disappears and discrete excitonic-like nanolevels occur within the forbidden energy gap
CdI2 single crystals are indirect and wide-bandgap semiconductors having layered structure, space group C6v4 , with highly anisotropic chemical bonds The band structure calculations of the CdI2crystals have also shown [2 4] a large anisotropy in the space charge density distribution causing high anisotropy in the corresponding optical spectra The anisotropic behaviour of the CdI2 crystals favours the local noncentrosymmetry, making them be able for the PSSHG investigations
Experimental as well as theoretical investigations per-formed in pure CdI2single crystals in the last few years using nonlinear spectroscopy have shown that CdI2 pos-sesses higher-order optical nonlinearities [5 8] An investigation for the magnetic field stimulated ferroelec-tricity in CdI2–Cu has also been reported [9] However, this measurement was preliminary performed a decade ago and the most recent report for this system is rare [10,11] Here we study Cu-doping effects in CdI2nanocrystals experimentally We use the photostimulated second har-monic generation (PSSHG) as a tool to investigate the effects It is found that the PSSHG increases with increasing Cu content up to 0.6% and then decreases due to the formation of the Cu-agglomerates, suggesting that a crucial role of the Cu-metallic agglomeration is involved in
Nanoscale Science and Technology Centre, Griffith University,
Nathan, Brisbane, QLD 4111, Australia
e-mail: m.miah@griffith.edu.au
M I Miah
School of Biomolecular and Physical Sciences, Griffith
University, Nathan, Brisbane, QLD 4111, Australia
M I Miah
Department of Physics, University of Chittagong, Chittagong
4331, Bangladesh
Nanoscale Res Lett (2009) 4:187–190
DOI 10.1007/s11671-008-9215-4
Trang 2the processes The PSSHG for the crystal with Cu content
higher than 1% reduces to that for the undoped CdI2
crystal
Experimental Details
Investigated samples are taken from 0.8 to 10 nm thick
crystals of CdI2 doped with Cu as well as undoped
Cu-doped CdI2 nanocrystals were synthesized from the
mixture of CdI2and CuI using standard
Bridgman-Stock-barger method Structure was monitored using an X-ray
diffractometer and the homogeneity was controlled using a
polarimeter The nanocrystal sample thickness was
con-trolled using a radio-frequency interferometer, using
conventional fringe-shift technique as discussed in details
in Ref [12] The investigation was performed at liquid
nitrogen temperature by mounting the samples in
temper-ature-regulated cryostat We used a Nd:YAG laser, as a
fundamental laser for the PISHG, which generates
pico-second pulses (average power 15 MW) with a repetition
rate of 80 mHz The output PSSHG (k = 530 nm) and
fundamental (k = 1,060 nm) signals were spectrally
sep-arated by a grating monochromator with a spectral
resolution of *5 nm mm-1 Detection of the
doubled-frequency (in the green spectral region) output PSSHG
signal was performed by a photomultiplier (with time
resolution about 0.5 ns), with an electronic boxcar
inte-grator (EBI) for the registration of the output During
evaluation of the time-delayed nonlinear optical response,
we measured the light intensities at the fundamental (x)
and doubled-frequencies (2x) with time steps of *50 fs
using the EBI in the time-synchronized pump-probe
con-ditions The second-order effective susceptibilities were
calculated using the relation [13]:
I 2x; tð Þ ¼2l
3=2
0 e3=20 x2l2
A
v2 ijkI2ðx; t sÞ
n 2xð Þn2ð Þx
sinlDk t2ð Þ
lDk t ð Þ 2
" #2
;
where l is the length of the nonlinear medium, i.e the
crystal thickness, l0and e0are the magnetic and dielectric
static (in vacuum) susceptibilities, respectively, A is the
area of the pumping beam which processes Gaussian-like
form, n(x) and n(2x) are, respectively, the refractive
indices for the pumping and PSSHG doubled frequencies,
vijk are the components of the second-order nonlinear
optical susceptibility determined from different angle of
the incident light and Dk¼ k 2xð Þ 2k xð Þ is phase
matching wave vector factor defined by photostimulated
birefringence The light intensities of the time-dependent
pumping I(x,t) and frequency doubled PSSHG signals
I(2x,t - s) were measured for different times (t) of pulse
duration and for different delaying times (s)
Results and Discussion The pumping power density dependence of the PSSHG signal was measured Figure 1 shows the results for a crystal (0.8 nm) As can be seen, the PSSHG increases with increasing power density and then decreases to a value a little higher than background after reaching a maximum The PSSHG dependence also shows a beginning of slight increase However, a significant enhancement in PSSHG occurs for the nanocrystal The qualitative and quantitative changes that occurred for the nanocrystal correspond to the manifestation of the quantum-confined excitonic levels perpendicular to the layer
Figure2shows the pump-probe delay dependence of the PSSHG signal for a typical sample (1.2 nm; 0.8% Cu) As can be seen, the relaxation time of the signal is relatively large Such a relaxation time is typically for the relaxation
of a particular layer in layered crystals, where a significant contribution from the interlayer rigid phonons might be involved [14] The relatively large relaxation time observed in the PSSHG pulses demonstrates the principal role of long-lived electron–phonon states in the observed effects explained within a model of photostimulated elec-tron–phonon anharmonicity [15], where the relaxation time for the thin nanolayers should be larger than for the strong localized electron–phonon states due to the nanosized effects
The second-order susceptibility determining the PSSHG
as a function of sample thickness for a doped sample (0.8 %) is shown in Fig.3 As can be seen, the PSSHG decreases with increasing the thickness of the crystal, and for the thickness higher than 10 nm, the PSSHG reduces to that for the undoped crystal (Fig.4), demonstrating that a significant enhancement is achieved for the 0.8 nm thick crystal
Power density (TW m-2)
-4 0 4 8 12 16 20
sample
Trang 3Figure5 shows the second-order susceptibility as a function of Cu-doping density for a sample with crystal thickness 2.5 nm The second-order susceptibility depen-dence of the Cu-doping density for a thin sample (0.8 nm)
is shown in Fig.6 From Figs.5and6one can see that with increasing Cu content up to 0.6% the PSSHG significantly increases For the Cu content 0.6% the PSSHG achieves its maximum for a crystal with thickness 0.8 nm The inser-tion of the Cu impurities favours a stronger local electron– phonon interaction, particularly its anharmonic part, through the alignment of the local anharmonic dipole moments by the pumping light [9] As a particular role of the local electron–phonon anharmonicity is described by third-order rank tensors in disordered systems [10], the PSSHG is very similar to that introduced for the third-order nonlinear optical susceptibility, which has been confirmed
by observing the relatively large third-order susceptibility
of undoped CdI2single crystals [7] The local disordering
of the Cu agglomerates plays additional role in the nano-size-confined effects
The PSSHG is found to be decreased for Cu density higher than 0.6% This decrease of PSSHG with increasing
Cu content is caused by agglomeration of the Cu impurities that is typical of such kinds of layered crystals As dem-onstrated earlier [9], this can be understood in terms of the agglomerate chemistry The creation of the Cu agglomer-ates favours a reduction in the active electron–phonon centres, effectively contributing to the noncentrosymmetry
of the output charge density, as well as leads to the occurrence of metallic clusters which additionally scatter light, and consequently, suppresses the effect at higher Cu content through the limitation of the enhancement of the local hyperpolarizability for the Cu agglomerate as well as the corresponding nonlinear dielectric susceptibility From the above analysis, one can conclude that a crucial role of
ττ (s)
0.00 0.02 0.04 0.06 0.08 0.10
0
2
4
6
8
sample
Thickness (nm)
0.3
0.4
0.5
0.6
0.7
0.8
for a doped sample
Thickness (nm)
0.30
0.35
0.40
0.45
0.50
for the undoped sample
Cu doping (%)
0.36 0.38 0.40 0.42 0.44 0.46 0.48 0.50
for a sample with thickness 2.5 nm
Trang 4the metallic agglomerates was involved in the processes
and was responsible for the observed effects
Conclusions
Cu-doping effects in CdI2 nanocrystals were studied
experimentally using the PSSHG and the chemistry
responsible for the effects discovered It was found that the
PSSHG increases with increasing Cu content up to 0.6%
and then decreases due to the formation of the
Cu-agglomerates, suggesting that a crucial role of the metallic
agglomerates was involved in the processes The PSSHG
for the crystal with Cu content higher than 1% was found to
be reduced to that for the undoped CdI2crystal
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Cu-doping (%)
0.4
0.5
0.6
0.7
for a thin sample